a) Field of the Invention
This invention relates to a Nixe2x80x94P electroless-plated magnetic hard disc substrate made of an aluminum alloy which is a high capacity-type, such as those using a MR head or the like, and a process for manufacturing the magnetic disc.
b) Description of the Prior Art
A magnetic hard disc substrate can be generally obtained by punching a roll coil made of an aluminum alloy to prepare a blank, grinding the blank to remove flaws, waviness, and the like to obtain a substrate, then electroless plating the substrate with Nixe2x80x94P, finish-polishing the plated substrate, then forming a magnetic film by magnetic sputtering, and coating the magnetic film with an overcoat to obtain a sputtering media. In order to allow the surface roughness and the flatness to fall in prescribed ranges respectively in these steps, the grinding of the magnetic hard disc substrate (blank) prior to the Nixe2x80x94P electroless plating is performed by the following method: the substrate is put onto a polishing board to which a nonwoven fabric polishing cloth made of an organic polymer is applied and (a) the polishing operation is performed under a fixed pressure while supplying, to the polishing face, a polishing solution produced by dispersing metal oxide particles, such as alumina, titania, and zirconia with an average grain size of 0.3 to 5 xcexcm, in an organic acid-type etchant wherein the polishing operation consists of a first stage polishing using abrasive grains of a larger size and successively second stage polishing using abrasive grains of a smaller size; or (b) the polishing operation is performed using a polishing solution produced by dispersing colloid particles of silica, zirconia, titania, or the like, with an average grain size of 0.01 to 0.3 xcexcm, in an acid-type or alkali-type etchant.
In general, the magnetic hard disc substrate made of an aluminum alloy which is electroless plated with Nixe2x80x94P and is used for a hard disc of memory devices such as a computer, is improving in recording density from year to year. There is a great demand for more improvement in the recording density. There is also a tendency to a higher density and larger capacity. It is therefore important to finish the magnetic hard disc substrate used in these fields so that the substrate has the prescribed surface roughness and flatness. Particularly, the realization of high density recording is due to a remarkable progress in low floatation technologies based on the improvement of the head, which requires the reduction of the interval between the head and the medium. With the reduction in the interval, the magnetic disc needs to have a smooth surface and mostly reduced surface defects. Specifically, the following polishing qualities are required: the surface roughness Ra less than 5 angstroms, the surface roughness Rmaxxe2x89xa680 angstroms, and the surface is free from scratches with a depth of 50 angstroms or more and from pits with a depth of 50 angstroms or more. In the conventional method in which the polishing is performed using a polishing solution containing abrasive grains composed of a metal oxide such as alumina, titania, or zirconia, particles with a large grain size cannot be prevented from getting slightly mixed therein during a classification stage because these oxides are produced by grinding a massive raw material and classifying the ground material. The contaminant particles with a large grain size cause the production of scratches with a depth of 50 angstroms.
In the method (a) among the aforementioned conventional methods, though the waviness of the substrate decreases, the obtained value of Ra is about 10 angstroms and polishing flaws with a depth as long as 100 xcexcm or more remains. In the method (b), in turn, the obtained value of Ra is about 3 angstroms and the polishing flaws decrease in depth to about 50 angstroms or less, but the waviness of the substrate remains. Also, the polishing speed is low, which requires a polishing time as long as about 5 minutes or more to attain the object Ra value. Magnetic hard disc substrates obtained after the polishing step in the conventional methods inevitably have the features that the surface roughness Ra is in a range from 7 to 15 angstroms, the surface roughness Rmax is in a range from 80 to 150 angstroms, and several scratches with a depth of 80 to 150 angstroms, some pits with a depth of 100 angstroms or less, and micro-waviness are produced. When using a suspension of known micro-sized and uniform silica particles to polish in order to avoid this situation, it takes a long time to polish because the particles are small. If erosive chemicals are added to accelerate the polishing, the silica will gel to-cause the silica particles to lose their uniformity. Thus a surface with the surface roughness Ra xe2x89xa65 angstroms and the surface roughness Rmaxxe2x89xa680 angstroms cannot be achieved at present.
It is an object of the present invention to solve the above prior art problems and to provide a magnetic hard disc substrate with a smooth surface and decreased defects which makes it possible to achieve high density recording and also to provide a process for manufacturing the magnetic disc.
Another object of the present invention is to provide a magnetic hard disc substrate which has a surface roughness Raxe2x89xa65 angstroms and is free from scratches (grinding flaws) with a depth of 50 angstroms or more and from micro-waviness.
A further object of the present invention to provide a magnetic disc which has a surface roughness Raxe2x89xa65 angstroms and a surface roughness Rmaxxe2x89xa680 angstroms and is free from scratches with a depth of 50 angstroms or more and from pits with a depth of 50 angstroms or more.
A still further object of the present invention is to provide a process for manufacturing the magnetic hard disc substrate in a short time and in a simple manner.
A still further object of the present invention relates to the above process using silica abrasive grains and is to provide a process for manufacturing the magnetic hard disc substrate having excellent surface properties in an efficient manner by adding, to the polishing agent, additives for avoiding gelation and for improving the rate of polishing.
Still further objects will be clear from the descriptions herein below.
The present invention resides to a polishing process in succession to a Nixe2x80x94P electroless plating process in a process for manufacturing an aluminum alloy magnetic hard disc substrate and comprises: dividing the polishing process into plural polishing stages, polishing a subject material using a polishing solution containing metal oxide abrasive grains with an average grain size of 0.3 to 5 xcexcm in a polishing stage preceding a final polishing stage, and polishing the treated subject material using a polishing solution containing colloid particles with a grain size of 0.01 xcexcm to 0.3 xcexcm in the final polishing stage. It is noted that the polishing process using a metal oxide abrasive grains, which is followed by the final polishing process, may be divided into a plurality of polishing steps using particles with different grain sizes respectively.
According to a further aspect of the present invention, there is provided a process for manufacturing a magnetic hard disc substrate comprises: processing a subject material until the surface roughness Ra is 15 angstroms or less and the surface roughness Rmax is 200 angstroms or less and then polishing the treated subject material using a colloidal abrasive agent containing silica particles with a grain size of 0.5 xcexcm or less, to which are added additives for preventing gelation and accelerating the rate of polishing.
Preferably the above additive contains 0.01 mol/l of a trivalent iron ion as an inorganic acid salt or organic acid salt in the present invention.
Other examples of the additive include 0.1 to 2.0 mol/l of hydroxyacetic acid, 0.1 to 2.0 mol/l of molybdenum oxide (hexavalent), 0.01% or more of hydrogen peroxide, 0.03 to 4.0 mol/l of aluminum nitrate, 0.03 to 2.0 mol/l of nitric acid, or a mixture thereof.
In the present invention having the above structure, a disc substrate which has a surface roughness Raxe2x89xa65 angstroms and is almost free from abrasive flaws with a depth of 50 angstroms or more and micro-waviness can be obtained.
In addition, a disc substrate having the polished qualities, in which the surface roughness Ra is 5 angstroms or less and the surface roughness Rmax is 80 angstroms or less and the number of scratches with a depth 50 angstroms or more and pits with a depth of 50 angstroms or less are both reduced to 5 or less, can be obtained.
The present invention will be explained in detail.
A first process of the present invention comprises a polishing process using abrasive grains composed of a metal oxide, the polishing process being carried out prior to the final polishing process. In the polishing process prior to the final polishing process, a subject substrate is polished using a polishing solution containing abrasive grains of a metal oxide such as alumina, titania, zirconia, or the like, the abrasive grains having a grain size of 0.3 to 5 xcexcm. When the average grain size of the metal oxide abrasive grains is less than 0.3 xcexcm, the polishing rate is low and hence it takes a long polishing time to reach the intended polished finish, though the surface roughness of the subject substrate is improved. On the other hand, when the average grain size exceeds 5 xcexcm, the polishing rate is high but the surface is more roughened and abrasive flaws increases. Incidentally, this polishing process may be carried out in a plurality of polishing stages. Illustrating, for example, a case of 2 stages, the process includes a first stage of polishing using metal oxide abrasive grains with an average grain size of 0.5 to 5 xcexcm and a second stage of polishing using metal oxide abrasive grains with an average grain size of 0.3 to 1.5 xcexcm. In this case, the second polishing stage is for a treating process preceding the final polishing process.
The substrate which has been roughly finished to the given precision and treated so as to be free from waviness and plating flaws is then subjected to the final polishing process. This final polishing process is performed using, as the polishing material, a polishing solution containing colloid particles of silica, titania, zirconia, or the like with an average grain size of 0.01 to 0.3 xcexcm to restrain the production of abrasive flaws to the extreme. If the average grain size of the colloid particles of silica, titania, zirconia, or the like is smaller than 0.01 xcexcm, the polishing process requires a long polishing time so that it is unpractical. On the other hand, if the average grain size of the colloid particles exceeds 0.3 xcexcm, the surface roughness Ra of the magnetic hard disc substrate is more than 50 angstroms.
The metal oxide abrasive grains used in the polishing process preceding the final polishing process is dispersed in an aqueous solution in a proportion of 1 to 40%. The aqueous solution is preferably adjusted to an acidic range of a pH of 2 to 6 using nitric acid, phosphoric acid, sulfamic acid, or the like. The colloid particles used in the final polishing process is dispersed in an aqueous solution in a proportion of 1 to 40% similarly. The aqueous solution is preferably adjusted to an acidic range of a pH of 2 to 6 using nitric acid, phosphoric acid, sulfamic acid, or the like or to an alkaline range using sodium hydroxide.
The abrasive grains used for the polishing in each stage gradually decrease in size with an increase in the number of stages. The polishing in each stage may be continuously performed using the same polishing machine and abrasive grains with different sizes. It is however desirable that each different polishing machine be used every polishing stage and the substrate be washed every time when the polishing in each stage is completed to prevent the contamination of the abrasive grains used in the antecedent stage. These devised measures make it possible to prepare a magnetic hard disc substrate which has a surface roughness Raxe2x89xa65 angstroms and is free from abrasive flaws with a depth of 50 angstroms or more and from micro-waviness.
Another embodiment of the method of the present invention comprises a step of processing the substrate using an abrasive agent of alumina, for example, with an average grain size of 0.3 to 2.0 xcexcm according to a known method until the surface roughnesses Ra and Rmax become 15 angstroms or less and 200 angstroms or less and, preferably 10 angstroms or less and 100 angstroms or less, respectively, and a step of polishing using a colloidal abrasive agent containing an additive possessing gelation preventive and polishing promoting activities and silica particles with a grain size of 0.5 xcexcm or less. Specifically, because it takes a long polishing time to obtain a substrate with the prescribed roughness only by using a colloidal abrasive agent containing silica particles, first the substrate is processed using an abrasive agent of alumina or the like, for example, with a grain size of 0.3 to 2.0 xcexcm, until the surface roughnesses Ra and Rmax become 15 angstroms or less and 200 angstroms or less and, preferably 10 angstroms or less and 100 angstroms or less respectively. After that, the substrate is polished using a colloidal abrasive agent containing silica particles. The silica particles used in the present invention are preferably a silica sol prepared by a known preparative method in which the silica sol is produced from an aqueous silicic acid solution. Specifically, the final polishing process is performed using spherical or spheroidal silica abrasive grains with a uniform grain size of 0.5 xcexcm or less and, preferably, 0.05 to 0.3 xcexcm, which cannot be attained by pulverization or classification. A prescribed amount of additives is added to the abrasive agent used in the polishing process using silica for the purpose of preventing gelation and outstandingly improving the polishing rate, which is a significant feature of the present invention.
The aforementioned additive used in the present invention is any one of the following materials:
(a) 0.01 mol/l or more and preferably 0.01 to 3 mol/l of a salt of a trivalent iron and an inorganic acid or organic acid; wherein examples of an inorganic acid include hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid; examples of the organic acid include citric acid, acetic acid, and oxalic acid; the amount of each of these inorganic acids or organic acids is controlled in such a manner that it is sufficient to form a salt but not excessive to avoid the existence of free ions of the acid. If a salt containing less than 0.01 mol/l of trivalent iron ions is added, the effect of iron ions is insufficient, whereas if the amount of trivalent ions exceeds 3 mol/l, the addition of the iron ions rather imparts an adverse effect.
(b) 0.1 to 2.0 mol/l of hydroxyacetic acid; if the amount of hydroxyacetic acid is less than 0.1 mol/l, no effect of the addition can be produced whereas an amount exceeding 2.0 mol/l rather imparts an adverse effect.
(c) 0.1 to 2.0 mol/l of molybdenum oxide (hexavalent); preferably 0.1 to 1 mol/l of molybdenum oxide (hexavalent) is added; and if the amount of molybdenum oxide is less than 0.1 mol/l, only an insufficient effect of the addition can be produced whereas an amount exceeding 2.0 mol/l rather imparts an adverse effect.
(d) 0.01% or more of hydrogen peroxide, preferably 0.01 to 40%, and more preferably, 0.01 to 30% of hydrogen peroxide, is added; and if the amount of hydrogen peroxide is less than 0.01%, only an insufficient effect of the addition can be produced whereas an amount exceeding 40% rather imparts an adverse effect.
(e) 0.03 to 4.0 mol/l of aluminum nitrate, preferably 0.1 to 2.0 mol/l of aluminum nitrate is added; and if the amount of aluminum nitrate is less than 0.03 mol/l, only an insufficient effect of the addition can be produced whereas an amount exceeding 4.0 mol/l rather imparts an adverse effect.
(f) 0.03 to 2.0 mol/l of nitric acid, preferably 0.1 to 1.0 mol/l of nitric acid is added; and if the amount of nitric acid is less than 0.03 mol/l, only an insufficient effect of the addition can be produced whereas an amount exceeding 2.0 mol/l rather imparts an adverse effect.
In this manner, according to the present invention, a disc substrate having the polished qualities in which the surface roughness Ra is 5 angstroms or less and the surface roughness Rmax is 80 angstroms or less and the numbers of scratches with a depth 50 angstroms and pits with a depth of 50 angstroms are both reduced to 5 or less can be obtained.
Also, according to the present invention, a disc substrate having the above polished qualities can be produced in a short time in a simple manner. Thus, a magnetic hard disc substrate can be obtained which can correspond to a reduced interval between a head and a medium, the reduction of the interval resulting from the low floatation of the head with an improvement in high density recording.